How do Septic Systems Work?

Below is a cross section diagram and simple explanation of a septic tank and how it works.

Septic tanks and septic fields are the most common domestic wastewater treatment systems used in rural areas.

Septic tanks remove settable and floatable solids from wastewater. Septic fields then get rid of the cleaned wastewater by letting it adsorb into the soil through perforated pipes. Removing solids from the wastewater protects the septic field from clogging and failure. Septic tanks also promote biological digestion of a portion of the solids and store the remaining undigested portion.

One side of the septic tank removes solids by holding wastewater in the tank. This allows heavier solids to settle as sludge and lighter particles (ie. grease) to form a scum layer in the top. To accomplish this, wastewater should be held in the tank for at least 24 hours. Up to 50% of the solids retained in the tank decompose. The remainder accumulates in the tank.

You can see in the diagram, wastewater volume is the “available space” between the floating scum and settled sludge. As the septic tank is used the scum and sludge continue to accumulate, making the wastewater volume smaller and smaller. This also reduces the amount of time the wastewater has to settle out as scum or sludge, leaving more suspended solids to flow out to the septic field.

If too many solids are left to go into the septic field it will shorten its life and cause failure by clogging the soil around the field pipes. To prevent this, the septic tank must be pumped on a proper schedule. If you don’t have a septic field and just have a pump out above ground, suspended solids are not as much of a concern but can still do damage to the pump. To go any more than 3 years between pump outs is not recommended. The scum layer hardens when it is exposed to the atmosphere and gets to a point where it can never be broken up and removed. Also, the H2S gas content will get so high that it will corrode the tank, shortening its life.

The schedule to pump out a septic tank is totally dependent on the volume of wastewater used per day and the size of the tank. The majority of tanks are 750 gallons so it mostly depends on the number of people in the house. Rule of thumb: 5-6 occupants need the tank pumped once a year, 3-4 occupants every 2 years, and 1-2 occupants once every 3 years. To be careful, many people have it pumped out every year. Those who do have the least amount of problems with their septic system.